Process for recovery of platinum from perchlorate cell electrolyte



PROCESS FOR RECGVERY OF PLATINUM FROM PERCHLORATE CELL ELECTROLYTE Joseph Heitrnan, Tacoma, Wash, assignor to Pennsalt Chemicals Corporation, a corporation of Pennsylvania No Drawing. Application July 13, 1953 Serial No. 367,731

9 Claims. (Cl. 75-108) This invention relates to a new method of recovering platinum from solutions or suspensions in which platinum is present in only a very small concentration. Platinum has heretofore been recovered from mixtures in which it is present in a minor amount by forming a soluble compound of platinum, for example, a chloroplatmate, and thereafter precipitating metallic platinum either by electrolytic means or by reduction with a metal such as zinc or magnesium. These methods are not efiictent when the platinum is present in a quantity less than 0.01% by weight of the solution and are essentially mefiective when the platinum is present in still smaller amounts.

The electrolysis of sodium chlorate to sodium perchlorate is an example of a process where the present invention finds ready application. In this process, it is the present practice to use platinum anode surfaces in the electrolytic cells and, While platinum is almost completely inert to most acids, with the exception of aqua regia, there is a slow loss of platinum from anode surfaces during perchlorate electrolysis. This loss is both chemical and physical and amounts to from to grams of platinum per ton of sodium perchlorate produced, dependmg upon the conditions of electrolysis and the physical condition of the anodes. The platinum from the chemical dissolution is probably present in the liquor either as chloroplatinic acid (H PtCI or as sodium chloroplatinate. Platinum also may be lost as a result of physical erosion by gas bubbles and liquor and in this case the platinum is in the form of very fine colloidal particles. There are also larger pieces of platinum foil present, resulting from mechanical breakage of anodes which have been worn thin.

In accordance with the present invention, it has been found that chloroplatinates are either adsorbed on or will react with insoluble hydroxides of the fiocculent or gelatinous type, such as the hydroxides of iron, nickel, cobalt and aluminum, and that this reaction or adsorption is substantially more complete when carried out in the presence of alkaline earth metal cations of group two of the periodic table. Examples of these are the cations of barium, strontinum, and calcium.

Generally speaking, in the treatment of sodium perchlorate solutions, or other solutions containing platinum in very small amount, for the recovery of the platinum, a flocculent metal hydroxide precipitate, such as ferric hydroxide, may be formed in the sodium perchlorate solution. Ferric hydroxide is admirably suited for this purpose since there is some ferric hydroxide already present in perchlorate cell liquor due to corrosion of the cells, and ferric hydroxide is also easily separated from the perchlorate solution by filtration. If insuflicient iron is present in the electrolyte for the precipitation of the platinum, additional iron may be added in the form of ferric chloride, which may be subsequently converted to ferric hydroxide by the addition of sodium hydroxide to the solution.

A cation as above defined must also be present in addiice dichromate. Since chromates are soluble to some extent.

in mineral acids, it is necessary that an alkaline condition be established before precipitation.

Ferric hydroxide is precipitated together with barium chromate upon adjustment of the pH of the solution to a value above 7. One of the results of the barium chloride. addition, as previously noted, is the precipitation of chromate, but it has been found that only a very small amount of platinum is precipitated with the ferric hydroxide, or other flocculent precipitate, unless an excessv of barium chloride, or other metallic cation, is present over that stoichiometrically required to precipitate the chromate. it is necessary to add the barium chloride, or other metallic cation, until the solution becomes water white, and this end point can be clearly seen in a strong light.

The co-precipitation may involve either a simple adsorption or a chemical reaction, but it is believed that in the case of the barium chloride-ferric hydroxide system, for example, a true chemical reaction takes place. This is indicated by the fact that ferric hydroxide will turn a brighter red color it a very small amount of a chloroplatlnate salt is added to it in an aqueous solution. it 18 possible, therefore, that a compound of the general formula BaPtCl -re(Ol-l) -yl-l t) is formed. The reaction which forms this compound appears to be somewhat reversible, however, since clnoroplatmates have been observed to go back into solution rrom a treated cell liquor. 'lhis reversal of the reaction may be prevented by immediate filtration of the sludge or by having a greater amount of barium chloride present.

After separating the sludge trom the perchlorate liquor, the sludge is boned, in the form of a thick slurry, with aqua regta until all of the metallic platinum is converted to chloroplatmates and all of the chromate is converted to trivalent chromium. A treatment sutflclent to accomplish the latter Will also be sum'cleht to accomplish the former. The residue trom this treatment is then' baked at an elevated temperature in order to convert all of the chloroplaunates to soluble salts which are believed to be chloroplatinites. 'lhe chloroplatihltes may then be removed trom the baked residue by boning or leaching the residue with hot water containing a small quantity of hydrochloric acid.

1he hot leach liquor is filtered to remove any suspended solid particles ot residue and is then made alkaline to precipitate the hydroxides of iron and chromium. These precipitates are then liltered oh, leaving a pale yellow SOlLlLlOIl of sodium, barium or other chloroplaunite. The color of this solution is dependent to a large extent on the pH thereof and becomes much paler at pH values below 7. The color of the solution is not an indication ot the amount of platinum present.

Since the final liquor contains barium chloride and probably other dissolved salts, the platinum salts cannot be isolated by simple evaporation. Platinum, being a noble metal, is easily reduced by other metals, such as zinc, cadmium, magnesium, or aluminum in an acidified SOlUllOll.

in a preferred mode of operation, platinum may be recovered trorn sodium perchlorate cell liquor by the following procedure; Sutlictent terric chloride is added to the perchlorate cell liquor to bring the terric ion concentration to a value equivalent to about 0.5 gram per liter of ferric chloride, and the pH of the solution is adjusted i toabdut 8.0 withsodium hydroxide. Ari-excess of barium chloride over that necessary to precipitate the ch'romate in the cell liquor is then added. The barium chloride,

f enerally' added as the dihyd'rate, requirement of the 7 "quor is about 2 to 3 grams per liter. The point at Which th'e platinum'has precipitated may be determined'hy a fi ation of a small sample and examination thereof for a water-white color. 'Chloro'p'latinates are very; highly colored and small amounts thereof will lend a yellow'tinge to aqueous solutions. This-color may be detected by careful visual examination. "Precipitationof'chromate and ferric hydroxide'will be accelerated if the temperature of the liquor is raised to about 80 C. The precipitateis then filtered off and the sodium perchlorate liquofmay theujbe treated, if necessary, with sodiun rcarbonate to remove any excess barium. The Y fP P1tate, which1will amount to about 3 gallons ,of wet l g pe n of sodiumper'chlorate producedgis placed n an evaporating dish having two or threetimes asmuch q pt es is occupied by the precipitate,-,since there is qlnfiothing on boiling. The precipitate is'moistened j lih' aqua regia and boiled, with occasional stirring and j i'rd addition, until all of the chromate is reduced to trivalent chromium, as evidenced by a dark spinach-green color. Chromates must be removed as they interfere with the final reduction of the platinum. 1

The. residue is then transferred to an'oven and baked at a temperature in the range of about 300 to 350,CQfor' 'aboutfan hour after the inteiior of the residue becomes heated to, about 300 C; at least. Thebakedresidue is then ground to a powder and transferred to a --tanls con- V taining about 10 times thevolumeof the residuafli. e., a about 10 gallons, per ton of perchlorate produced, of hot water containing about 0.8% of hydrochloric acid, and the mixture is boiled for about 15; to ZO rninutes.

' The mixture is then filtered,'u sing a filter aidfiad WfiShed.

. The filtrate is retained.

,The' filtrate is made; slightly; alkaline, to. a .pH-of about.

8 .0, with sodium hydroxide BHd'IhQ resulting ferrictand "ehromic hydroxidesare'lilteredofi. The,filtrate,i,which t i is a pale yellow ;in color, is. retained and, madeslightly acid by. the additionof hydrochloric acid,; or other.acid.

. Amore reactive metal.than.platinum,.such as magnesium, theform of powder. or sheet, is-LaddecLto. theaacidic filtrate, and the1resuIting-rea'ction. ispermitted .to=proceed to.c ompietin,..as;5evidenced by..a.water=white:.color in the liquor; Additional metalzand' acid 'maystbe .added, if checessary, and; best results are obtainedhifi. the: mixture is agitated. The pH mustbe maintained-at:alltimeson 1 .the acid sidebut a-large excess-of. acid should be. avoided.

' Upon completion of the :reaction, the .liquor is brought to boiling temperature :to coagulatetheplatinum pafticles V andqto insure the dissolution of the:more-meactive-re- V dueirig metal; The. platinum, in thE- fOIITL' Of -a pure, 55

finely divided blaclc powder, is then:removed-trom the liquor by filtration. a f

It is understood thatthe-foregoing= description is illustrative only and variations thereinmay be made ifdesired.

For example, the a baking M temperature for-converting the chloroplatinates to chloroplatinites may bein the range of about 250 to 580-C. --*ancl the heating may be continned for about 0510* 8 hours. 4 similarlyy leachingof 1 the-baked residue may beweffected at-afitemper ature. of

platinum may be recovered -in met'allic form from 'the fsoluble salts thereof by'chemical or electrolytic reduction.

f The invention'willbe furtherillustrated'by reference to jthe' following specific examples:

V ifraniple 1 sme ma-me a sdlution' was repared c'ontaining -750 ramsof sodium perchlorate; rams bfisodiumchlorate, 0.fii.="s am of-ealciunr chloride; '1 gram'of sodium dichro- .mate, and 0.0104 gram of ;platinum, as an equivalent amount of potassium chloroplatinate.

0.5 gram of ferric chloride was dissolved in the solution and the solution was made alkaline with sodium hydroxide 5 to precipitate ferric hydroxide. Sufficient barium chloride was added to precipitate thechromate and then an excess of barium chloride was added until all ofthe platinum.

was reacted with. and/or adsorbed on the ferric hydroxide, as shown by an absenceof color in the solution-. The barium chloride dihydrate requirement was 3 grams. The solution-wasthenfiltered to remove the precipitate, which was then'boiled with aqua regia until all of the chromate was reduced, and-was then baked at a temperature of 300 to 350 C. for one hour to convert the platinum to 15 a soluble form,--believed'-to be chloroplatinite salts. The' residue was then ground and boiled in water, containing 0.8% hydrochloric acid, to dissolve solublesalts and was then filter'ed. -Thefilttate Was made alkaline'toremove The final filtrate contained solublechloroplatinite salts, I from'which the-'platinumwas recovered in metallic form by acidifyingthe-filtrate-and reducing the resulting acid compounds. ofpla'tinum by the addition of magnesium sheet. The recoverywf' metallic platinum was 7 0.0092 gram, which is-equiv'alent to an 89% recovery of the platinum in solution. f V I litersofi sodium :perchlorate electrolyte, :produced by the electrolysis of sodium chlorate to sodium perchlorate, andhayinga compositiongsimilar 2tothattof thesoluplatinum or platinum; salts,fromLperchloiatexell? liquor and other dilute solutions. tAlso,-I in;the prescntprocess only a slight; quantity ofadditional materialstisadded to t the cell,.liquonghence'theretisfno. contamination thereof and consequently no extra purification steps are: required. It will be obyiou's to: those;skilled intheartithat many modificationsmayzbe madezwithin the scoperof the present invention withoutide'parting fromthe spirit'thereofi and i a 1 the-invention includes; alli suchmodtfications;

.. I-;cl'aim: P 1 i j .1. .Thezlmethod .ofurecoveringplatinum from perchlo- 1 .rate .cell electrolyte;- whichlcomprises forming aflocculent .:metal; hydroxide;precipitate in.thezsaid electrolyte in' the 7 presence of an alkalinezearthlmetal cation. addedin 0 l excess-mot the.- stoichiometric. amount; .requiredto pre:

. cipitate-thei anions present intthexsaidelectrolyte, which i arereactiverwith. .alkaliaearth metal-cations, separating I the resulting precipitate; andrecovering.metallicplatinum :therefrom; 7 a V t 2*.1The *proc'essiofgclaitnl.1::in which:,-the .metal hy- "f.dro'xide-.is selectediifroirnthe; groupzconsisting of ferric, 2 nickel; cobalt and; aluminum hydroxides.

3; .The methodgof, claim 1 in which the metal hya :droxideljslfe'rric hydroxide:

:4. ,Iliezmethod acclaim-1; inwhich the alkaline earth cmetal cation -i'srbarium. t t a a.

f;.5.-z:Thezmethod;ofirecovering platinum from perchlo- 5;" rate; cell-5' electrolyte; .contjainingzminute; amountsofplatinum as well as small mounts of chromateion which ..comprises adding sallialii to adjust'the;v pH; of the; said hydroxides of iron and-'chromium and wasagain filtered.

,tion of. Exampledexcept-for the quantity of platinum,

electrolyte to in excess of 7, adding alkali earth metal cations in excess of the amount necessary to precipitate all of the chromate present in the said electrolyte, forming flocculent metal hydroxide precipitate in the presence of an excess of the said alkali earth metal cation, separating the resulting precipitate from the liquor and recovering metallic platinum therefrom.

6. The method of claim 5 in which the alkali earth metal cation is barium.

7. The method of claim 5 in which ferric chloride is added to the perchlorate cell electrolyte until its concentration is about 0.5 gram per liter, the said addition being made prior to the addition of the alkali earth metal cation.

8. The method of claim 5 in which the addition of the alkali metal cation is continued until a filtered portion of the treated electrolyte is water-white in color.

9. The method of recovering platinum from perchlorate cell electrolyte containing minute amounts of platinum as well as small amounts of chromate ion which comprises adding alkali to adjust the pH of the said electrolyte to an excess of 7, adding an alkali earth metal cation in excess of the amount necessary to precipitate all of the chromate present in the said electrolyte, and forming flocculent metal hydroxide precipitate in the presence of an excess of the said alkali earth metal cation.

References Cited in the file of this patent UNITED STATES PATENTS 1,797,879 Rudolfs Mar. 24, 1931 1,876,943 Hull Sept. 13, 1932 2,086,384 Lady July 6, 1937 2,131,045 Hickman Sept. 27, 1938 2,344,429 Gelder Mar. 14, 1944 2,542,743 Weymouth Feb. 20, 1951 OTHER REFERENCES Kolthofi et al.: Textbook of Quantitative Inorganic Chemistry, The MacMillan Co., New York, rev. ed. (1946).

Sandell: Colorimetric Determination of Traces of Metals, Interscience Publishers, 1110., New York, 2nd ed. (1950), pages 20-27 and page 494. (Copies of above pub. in Sci. Lib.) 

1. THE METHOD OF RECOVERING PLATINUM FROM PERCHLORATE CELL ELECROLYTE WHICH COMPRISES FORMING A FLOCCULENTP METAL HYDROXIDE PRECIPITATE IN TE SAID ELECTROLYTE IN THE PRESSURE OF AN ALKALINE EARTH METAL CATION ADDED IN EXCESS OF THE STOICHIOMETRIC AMOUNT REQUIRED TO PRECIPITATE THE ANIONS PRESENT IN THE SAID ELECTROLYTE, WHICH ARE REACTIVE WITH ALKALI EARTH METAL CATIONS, SEPARATING THE RESULTING PRECIPITATE, AND RECOVERING METALLIC PLATINUM THEREFROM. 